Actually, even though the daytime temperatures around Seattle are now into the (gasp!) mid-high 70's, Propel is still pumping their "winter mix" which is 50-75% bio rather than 99-100%. But Westly loves it, and returns 28-30 mpg on my daily commute. The price on this resplendent spring morning was $3.99/gal. I could have had petro-diesel for $3.79 and saved a few dollars and sometimes I do get the 5% mix at Arco or Fred Meyer instead.
Here is why I'm willing to pay more: There are several sites around Seattle where anyone can drop off used cooking oil and local restaurants (including my favorite neighborhood eatery Endolyne Joe's where the Sunday night fried chicken is boffo) have collection tanks as well. I drive by General Bio-Diesel in South Seattle every day where this waste oil is converted to fuel. The 7-11 that sells Propel is just a few blocks away. That kind of close loop saves a tremendous amount of energy and waste compared to the world-wide transportation and distribution of petroleum based diesel. So maybe my 31 year old engine is not as clean burning as the new high tech mills are, but at least I feel like I'm doing something to help.